Blake McIver, who played Waldo, tweeted the picture. (You may also recognize him as one of Michelle Tanner's precocious friends on Full House.) He posted the image with the caption: "It's #Rascals20th day! Hooray!" How the Internet has not yet broken as a result of this is beyond us.
The whole thing probably makes you feel a range of emotions. You're likely having an "I'm so old" moment, and perhaps some pangs of nostalgia. We recommend just accepting it for the beautiful moment it is — and don't worry about whether or not that's the same dog.